For a portable terminal or personal digital assistant (PDA) such as a cellular phone, a more compact and more high-performance device is required. For this reason, there has been a concentrated effort to reduce the size of and to enhance the performance of a device such as a high-frequency device, incorporated in a PDA.
In portable terminals and wireless LANs (Local Area Networks), a GHz band is mainly used as a carrier frequency. Conventionally, a semiconductor device, which is used for analog circuits for GHz-frequency-band-using transceiver, has been formed on a gallium arsenide substrate.
However, the microfabrication technology of a silicon CMOS (Complementary Metal Oxide Semiconductor) has advanced, and the operational frequency on a silicon substrate goes high, thereby enabling a semiconductor device operated in a GHz frequency to be fabricated by use of a silicon substrate. The device can be fabricated at a lower production cost on the silicon substrate than on the gallium arsenide substrate. Moreover, when a silicon substrate is used, a digital circuit that has been fabricated on a silicon substrate and an analog circuit for transmission and reception can be advantageously fabricated on a single substrate.
Important passive elements used for the analog circuit include a spiral inductor. The spiral inductor is several tens to several hundreds of micrometers in diameter, and occupies an extremely large area as compared with an active element such as a transistor. Therefore, in order to reduce the size of a semiconductor device provided with an analog circuit, if the size of the spiral inductor can be reduced, it is very effective.
A conventional typical spiral inductor is composed of a spiral interconnection formed of an electroconductive film layer wired and disposed in a spiral shape, an underpass interconnect, which is led out to outside from the internal end of this spiral interconnect, and a plug interconnect, which electrically connects the spiral and underpass interconnects.
The spiral inductor is formed on an insulating film provided over the semiconductor substrate. For the electroconductive film layer, on a gallium arsenide substrate, for example, gold or gold alloy is used, and on a silicon substrate, for example, aluminum, aluminum alloy, or copper is used.
As described above, the microfabrication technique of the silicon CMOS enabled a silicon substrate to be used in the fabrication of a high frequency analog circuit that can be used in a GHz frequency band, thereby bringing forward the size reduction of a semiconductor device. However, in order to form fine patterns in a lateral direction in the semiconductor device, it is necessary to reduce the thickness of the electroconductive film layer used for the analog circuit. The thickness reduction of the electroconductive film layer can reduce the size of the spiral inductor; however, when the thickness of the film layer is reduced, the resistance of the spiral inductor increases, to thereby degrade the performance of the device.
For this reason, conventionally, as a practicable solution, the following method has been proposed.
For example, a conventional spiral inductor proposed by J. N. Burghartz, et al. (see “Microwave Inductors and Capacitors in Standard Multilevel Interconnect Silicon Technology,” IEEE TRANSACTION ON MICROWAVE THEORY AND TECHNOLOGYS, VOL. 44, and pp.100–104, 1996) is fabricated by using four-layer electroconductive film layers made of an alloy of aluminum and copper. A silicon substrate is used for the semiconductor substrate, and a first to fourth electroconductive film layers are stacked on an insulating layer provided over the substrate, with insulating layers each disposed between the electroconductive film layers. A spiral interconnect is formed by connecting these electroconductive film layers in parallel by use of a plug interconnect.
Thus, two or more electroconductive film layers are connected in parallel to form the spiral interconnect, thereby reducing the resistance of the spiral interconnect and enhancing the performance of the spiral inductor.
In this conventional spiral inductor, an underpass interconnect is formed by use of the first electroconductive film layer, and is electrically connected with the spiral interconnect via the plug interconnect.
Moreover, for example, in a conventional spiral inductor disclosed in JP-A-9-181264, a spiral interconnect is formed by use of a second electroconductive film layer, and an underpass interconnect is formed by use of a first electroconductive film layer located under the second electroconductive film layer. In addition, the area within the first electroconductive film layer, which is other than the area where the above underpass interconnect is formed, is used to form another spiral interconnect, and the obtained spiral interconnect is connected with the above spiral interconnect formed of the second electroconductive film layer in parallel.
Thus, in this conventional spiral inductor, of the spiral interconnect, in the portion where a plurality of electroconductive film layers are connected in parallel with each other, the thickness of the electroconductive film layer has been substantially increased, which can reduce the resistance of the portion.
However, in these conventional spiral inductors, because the underpass interconnect, and the spiral interconnect in the area where the spiral interconnect intersects the underpass interconnect in the above-mentioned second conventional example are formed with a small number of thin electroconductive film layers, the resistance of the portion cannot be reduced.
In addition, there arise the following problems when large electric current is passed through the spiral inductor.
When the spiral inductor is used for a circuit through which large current flows such as the transmitter circuit of radio communication, how to prevent the occurrence of an electromigration (breakage of the wiring) is a more important subject than how to enhance the performance of the spiral inductor. The electromigration is caused by the generation of defects, caused by the migration of metal atoms in the interconnect, which is caused by the electron flowing through the interconnect.
For the conventional spiral inductors mentioned above, in the portion that is formed with a small number of electroconductive film layers, the electromigration is easily caused. To be more specific, the underpass interconnect in the above spiral inductor described by J. N. Burghartz et al., and the underpass interconnect and the portion of the spiral interconnect, in which the spiral interconnect intersects this underpass interconnect, in the spiral inductor disclosed in JP-A-9-181264 are formed with a small number of electroconductive Film layers. Therefore, even if the spiral interconnect portion is formed with two or more electroconductive film layers, only the current, which can pass the underpass interconnect portion formed with one layer, can be passed through this spiral interconnect portion by reason of reliability.
In order to prevent the occurrence of the electromigration in these conventional spiral inductors, the following methods are assumed.
One method is to increase the width of the underpass interconnect, to thereby increase the resistance of the underpass interconnect portion to the electromigration. However, when the width of the underpass interconnect, which is close to the semiconductor substrate, is increased, the wider the width of the interconnect is, the larger the parasitic capacitance between the underpass interconnect and the semiconductor substrate becomes, which may lead to deteriorate the performance of the spiral inductor. Because the opposed area between the spiral interconnect and the underpass interconnect is also increased, the parasitic capacitance existing therebetween increases. This also can become a factor to degrade the performance of the spiral inductor.
Another method is to form the underpass interconnect by connecting in parallel the two electroconductive film layers, which are closest to the semiconductor substrate, and to form the spiral interconnect by use of only the top electroconductive film layer. According to this method, the underpass interconnect is composed of the two layers, which may enhance the resistance thereof to the electromigration. Further, this method can suppress an increase in the parasitic capacitance existing around the underpass interconnect as compared with the above-mentioned method. However, in order to obtain the same inductance as that obtained when the spiral interconnect is formed with two layers, it is necessary to increase the width of the interconnect, and to simultaneously increase the diameter of the spiral inductor. This requires a corresponding large area.
Thus, in order to prevent the electromigration in the conventional spiral inductors, the parasitic capacitance may increase to degrade the performance thereof or increase the area of the spiral inductor. In other words, there remains a problem that it is necessary to sacrifice either of the performance or the area.
The present invention has been accomplished to solve the above-mentioned problem, and an object of the present invention is to provide a spiral inductor such that the occurrence of the electromigration can be prevented while the performance thereof is maintained and small-sized.